Recently, training systems for training a subject utilizing the technology of computer graphics (CG) such as virtual reality (VR) are available (for example, see Japanese Patent Laying-Open No. 2004-294593 (hereinafter referred to as '593 Reference) and Japanese Patent Laying-Open No. 2007-264055 (hereinafter referred to as '055 Reference)). A training supporting apparatus disclosed in '593 Reference detects, as the biological reaction of the subject, an active region of his/her brain using near infrared light, to assist rehabilitation and image training of a subject with disabilities. The training supporting apparatus measures an active region or regions of the subject's brain while the subject is working on a calculation problem or a memory task imposed as a training material, and after the training, the effectiveness of the training is confirmed. '055 Reference discloses a training system that always keeps an optimal training scenario in accordance with the biological reaction of a subject during training.
Such a technique of scientifically grasping physiological indexes that would not otherwise be sensed and feeding these back to enable perception by the subject is referred to as “bio-feedback.” Though conventional bio-feedback sometimes utilizes biological information such as pulse and breath, it mainly involves converting human brain wave outputs into visible images or audible sounds and feeding them back to humans. A subject can grasp the state of his/her brain waves on real-time basis. Therefore, bio-feedback is helpful for the subject to control the state of his/her own brain waves.
Human sensory and esthesic systems are ever-changing in accordance with the surrounding environment. Most of the changes occur in a certain early period of human developmental stage, or the period referred to as a “critical period.” Adults, however, still keep sufficient degree of plasticity of sensory and esthesic systems to adapt to significant changes in surrounding environment. By way of example, it is reported, by Watanabe et al., that adults subjected to a training using specific esthesic stimulus or exposed to specific esthesic stimulus have improved performance for the training task or improved sensitivity to the esthesic stimulus, and that such results of training were maintained for a few months to a few years (T. Watanabe, J. E. Nanez Sr., S. Koyama, I. Mukai, J. Liederman and Y. Sasaki: Greater plasticity in Lower-level than higher-level visual motion processing in a passive perceptual learning task. Nature Neurosceience, 5, 1003-1009, 2002). Such a change is referred to as sensory learning, and it has been confirmed that such a change occurs in every sensory organ, that is, vision, audition, olfaction, gustation, and taction.
Nikolaus Weiskopf reports an example of bio-feedback applying fMRI (functional Magnetic Resonance Imaging), which is a method of visualizing hemodynamic reactions related to human brain activities utilizing MRI (Magnetic Resonance Imaging), rather than the brain waves (Nikolaus Weiskopf, “Real-time fMRI and its application to neurofeedback”, NeuroImage 62 (2012) 682-692). Further, Shibata et al. report one such type of feedback method, in which a stimulus as an object of learning is not directly applied to a subject while brain activities are detected and decoded and only the degree of approximation to a desired brain activity is fed back to the subject to enable “sensory learning.” (Kazuhisa Shibata, Takeo Watanabe, Yuka Sasaki, Mitsuo Kawato, “Perceptual Learning Incepted by Decoded fMRI Neurofeedback Without Stimulus Presentation”, SCIENCE VOL 334 9 Dec. 2011). Such a method of bio-feedback is referred to as DecNef method (Decoded NeuroFeedback method).
Nuclear Magnetic Resonance Imaging as such will be briefly described in the following.
Conventionally, as a method of imaging cross-sections of the brain or the whole body of a living body, nuclear magnetic resonance imaging has been used, for example, for human clinical diagnostic imaging, which method utilizes nuclear magnetic resonance with atoms in the living body, particularly with atomic nuclei of hydrogen atoms.
As compared with “X-ray CT,” which is a similar method of human tomographic imaging, characteristics of nuclear magnetic resonance imaging when applied to a human body, for example, are as follows:
(1) An image density distribution reflecting distribution of hydrogen atoms and their signal relaxation time (reflecting strength of atomic bonding) are obtained. Therefore, the shadings present different nature of tissues, making it easier to observe difference in tissues;
(2) The magnetic field is not absorbed by bones. Therefore, a portion surrounded by a bone or bones (for example, inside one's skull, or spinal cord) can easily be observed; and
(3) Unlike X-ray, it is not harmful to human body and, hence, it has a wide range of possible applications.
Nuclear magnetic resonance imaging described above uses magnetic property of hydrogen atomic nuclei (protons), which are most abundant in human cells and have highest magnetism. Motion in a magnetic field of spin angular momentum associated with the magnetism of hydrogen atomic nucleus is, classically, compared to precession of spin of a spinning top.
In the following, as a description of background of the present invention, the principle of magnetic resonance will be summarized using the intuitive classical model.
The direction of spin angular momentum of hydrogen atomic nucleus (direction of axis of rotation of spinning top) is random in an environment free of magnetic field. When a static magnetic field is applied, however, the momentum is aligned with the line of magnetic force.
In this state, when an oscillating magnetic field is superposed and the frequency of oscillating magnetic field is resonance frequency f0=γB0/2π (γ: substance-specific coefficient) determined by the intensity of static magnetic field, energy moves to the side of atomic nuclei because of resonance, and the direction of magnetic vector changes (precession increases). When the oscillating magnetic field is turned off in this state, the precession gradually returns to the direction in the static magnetic field with the tilt angle returning to the previous angle. By externally detecting this process by an antenna coil, an NMR signal can be obtained.
The resonance frequency f0 mentioned above of hydrogen atom is 42.6×B0 (MHz) where B0 (T) represents the intensity of the static magnetic field.
Further, in nuclear magnetic resonance imaging, using changes appearing in detected signals in accordance with changes in the blood flow, it is possible to visualize an active portion of a brain activated in response to an external stimulus. Such a nuclear magnetic resonance imaging is specifically referred to as fMRI (functional MRI).
An fMRI uses a common MRI apparatus with additional hardware and software necessary for fMRI measurement.
The change in blood flow causes change in NMR signal intensity, since oxygenated hemoglobin has magnetic property different from that of deoxygenated hemoglobin. Hemoglobin is diamagnetic when oxygenated, and it does not have any influence on relaxation time of hydrogen atoms in the surrounding water. In contrast, hemoglobin is paramagnetic when deoxygenated, and it changes surrounding magnetic field. Therefore, when the brain receives any stimulus and local blood flow increases and oxygenated hemoglobin increases, the change can be detected by the MRI signals. The stimulus to a subject may include visual stimulus, audio stimulus, or performance of a prescribed task, as disclosed, for example, in Japanese Patent Laying-Open No. 2011-000184.
In the studies of brain functions, brain activities are measured by measuring increase in nuclear magnetic resonance signal (MRI signal) of hydrogen atoms corresponding to a phenomenon that density of deoxygenated hemoglobin in red blood cells decrease in minute vein or capillary vessel (BOLD effect).
Particularly, in studies related to human motor function, brain activities are measured by the MRI apparatus as described above while a subject or subjects are performing some physical activity. The operation (task) to be done by the subject may include, for example, an operation of gripping some object. While the subject grips a detecting portion of a grip force detecting device, the force exerted on the grip force detecting device is detected and brain activities of the subject during the gripping operation are measured by the above-mentioned MRI apparatus.
For human subjects, non-invasive measurement of brain functions is essential. In this aspect, decoding technique enabling extraction of more detailed information from fMRI data has been developed (Kamitani Y, Tong F. Decoding the visual and subjective contents of the human brain, Nat Neurosci, 2005; 8: 679-85). The above-described DecNef is an application of such a decoding technique to a task related to sensory learning.